Iago claims he hates Othello because Othello passed him over for a promotion, giving Cassio the job instead even though Iago has more experience. Roderigo is jealous of Othello's relationship with Desdemona and agrees to help Iago bring down Othello. Bianca also becomes jealous when she finds the handkerchief in Cassio's room. Iago manipulates Othello's jealousy by telling him about the handkerchief, destroying Othello's mind and soul.

1. Jealousy

3. Act 3, Scene 3

4.  Iago claims he hates Othello because Othello passed
him, Iago, over for a promotion, giving "one Michael
Cassio" the job as his military lieutenant instead. Iago
claims he's far more qualified than Cassio, who lacks
Iago's experience on the field of battle.

5. One Michael Cassio, a Florentine,
A fellow almost damn'd in a fair wife;
That never set a squadron in the field,
Nor the division of a battle knows
More than a spinster; unless the bookish theoric,
Wherein the toged consuls can propose
As masterly as he: mere prattle, without practise,
Is all his soldiership. But he, sir, had the election:
And I, of whom his eyes had seen the proof
At Rhodes, at Cyprus and on other grounds
Christian and heathen, must be be-lee'd and
calm'd
By debitor and creditor: this counter-caster,
He, in good time, must his lieutenant be (1.1.2)

6.  The lovesick Roderigo has trouble with his hidden
feelings for Desdemona and is jealous watching the
two in love. Because of his love for Desdamona and
subsequent jealousy of Othello, Roderigo readily
agrees to help Iago in his plans to bring down Othello.

7.  Bianca, Cassio's mistress, also becomes sick with
jealousy. She sees the planted handkerchief in Cassio's
room and believes him to also have taken a lover. Her
jealousy exists on a much smaller scale, but illustrates
that the sentiment is universal.

8.  Othello is easily taken in by Iago’s lies and
manipulation because of his age, race, and because he
loves “too much”. When Iago tells Othello of the
handkerchief, he has the evidence necessary to prove
Desdemona's unfaithfulness. It is now that the
jealousy sinks deep into Othello's soul and starts to
destroy his mind.

9. I pray you, in your letters,
When you shall these unlucky deeds relate,
Speak of me as I am; nothing extenuate,
Nor set down aught in malice: then, must you speak
Of one that lov'd not wisely but too well;
Of one not easily jealous, but, being wrought,
Perplex'd in the extreme; of one whose hand,
Like the base Indian, threw a pearl away
Richer than all his tribe; of one whose subdu'd eyes
Albeit unused to the melting mood,
Drop tears as fast as the Arabian trees
Their med'cinable gum. Set you down this;
And say besides, that in Aleppo once, O! now, for ever
Where a malignant and a turban'd Turk Farewell the tranquil mind;
Beat a Venetian and traduc'd the state, farewell content! (3.3)
I took by the throat the circumcised dog,
And smote him thus. (5.2)